Most non-contact voltage testers will show a wire as hot even when it is off, if it is in a common conduit with other hot wires. The energized wires will induce enough voltage into the "dead" wire for the tester to detect. These types of testers often give a false positive (indicating voltage where there is none), but in some cases will do the opposite. The most common case where the tester will show no voltage where there really is voltage is NM cable where the paper within the jacket has become wet. This wet paper in contact with the bare EGC creates a shield and many times the tester will show no voltage even when the wire is live.
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